Serologic Observations and Serosurvey of Poxvirus Infections in California Sea Lions (Zalophus californianus)
Poxvirus infections are a common complication in the rehabilitation of stranded pinnipeds. As part of an ongoing study into this disease, an indirect ELISA was developed as a tool to study the epidemiological features of poxvirus infections of California sea lions both in rehabilitation centers and in the wild. For validation of the ELISA a total of 54 serum samples from 26 affected sea lions, collected opportunistically before disease, during the acute stage, and during the convalescent stage of the disease, were analyzed. A rise in circulating anti-poxviral antibodies was observed in all sea lions (n = 8) for which a convalescent serum sample (>1 day post first clinical signs) was available. The intensity and the time of onset of the humoral immune response varied, but a rise in circulating antibodies was detectable as soon as 2 days before first clinical signs (FCS) (n = 3) and in all convalescent sera (n = 8, up to 65 days after FCS). The immune response peaked around day 22 after FCS. In two sea lions, the rise in circulating antibodies could not be detected up to respectively 21 and 24 days after hospitalization, but had occurred by respectively 58 and 73 days after hospitalization. All serum samples collected prior to clinical disease (N = 21, mean days before FCS = 32 ± 24) were already considered positive. Additionally, 161 serum samples were collected at various timepoints from 74 California sea lions without pox lesions. A rise in circulating anti-poxviral antibodies was observed in 11 (15%) of these unaffected sea lions (mean increase = 3.1 ± 1.2, min 1.7, max 5.0) compared to samples collected within 6 days of admission. In one sea lion this rise occurred between 1 and 3 days after hospitalization, whereas in another unaffected sea lion the rise in antibodies was not yet detectable at 22 days, but had occurred by 59 days after hospitalization. Also, of these 11 sea lions, three were maintained at the rehabilitation center for 31-33 days after the rise in circulating antibodies was detected and no pox lesions were observed. Finally, to determine prevalence of antibodies to poxviruses in wild sea lion populations, serum samples were collected and analyzed from 761 sea lions distributed across all age categories. Overall, anti-poxviral antibodies were detected in 93.4% of the sera. The highest antibody prevalence (98.0%) was observed in 2-4 mo-old-pups, whereas the lowest antibody prevalence (84.3%) was observed in adult males (p < 0.05). From these studies, we can conclude that the presence of circulating antibodies does not correlate with protection and re-infection may therefore occur. Exposure to the virus did, in some cases, occur during hospitalization, and subclinical poxvirus infections do take place. It is unknown whether subclinically infected animals also shed virus, which would complicate the management of poxvirus outbreaks in rehabilitation centers. We can further conclude that poxviruses are endemic in wild populations and that introduction of poxviruses into the wild is therefore not of concern.